Sintering behaviour of alumina±niobium carbide composites W. Acchar a, *, P. Greil b , A.E. Martinelli c , C.A.A. Cairo d , A.H.A. Bressiani e , J.C. Bressiani e a Department of Physics, Universidade Federal do Rio Grande do Norte, Lagoa Nova Campus, Natal, RN, 59072-970, Brazil b Department of Materials Science (III) Glass and Ceramics, University of Erlangen-Nuernberg, D-91058 Erlangen, Germany c Department of Mechanical Engineering, Universidade Federal do Rio Grande do Norte, Lagoa Nova Campus, Natal, RN, 59072-970, Brazil d Divisa Äo de Materiais, Centro Te Âcnico Aeroespacial, Instituto de Aerona  utica e Espac Ë o, Prac Ë a Marechal do Ar Eduardo Gomes 50, Sa Äo Jose  dos Campos, Sao Paulo, SP, 12228-904, Brazil e Instituto de Pesquisas Energeticas e Nucleares, C.P. 11049 Pinheiros, Sao Paulo, SP, 05422-970, Brazil Received 14 September 1999; received in revised form 9 February 2000; accepted 15 February 2000 Abstract Ceramic cutting tools have been developed as a technological alternative to cemented carbides in order to improve cutting speeds and productivity. Al 2 O 3 reinforced with refractory carbides improve fracture toughness and hardness to values appropriate for cutting applications. Al 2 O 3 ±NbC composites were either pressureless sintered or hot-pressed without sintering additives. NbC contents ranged from 5 to 30 wt%. Particle dispersion limited the grain growth of Al 2 O 3 as a result of the pinning eect. Pres- sureless sintering resulted in hardness values of approximately 13 GPa and fracture toughness around 3.6 MPa m 1/2 . Hot-pressing improved both hardness and fracture toughness of the material to 19.7 GPa and 4.5 MPa m 1/2 , respectively. # 2000 Elsevier Sci- ence Ltd. All rights reserved. Keywords: Al 2 O 3 ; Composites; Cutting tools; Hardness; NbC; Sintering 1. Introduction Structural ceramics are characterised by high hot- hardness and superior chemical stability. Nevertheless, their brittle nature restrains their use as cutting tools to a present 5% of a consolidated market dominated by high-speed steels (AISI class T and M), and cemented carbides (Co-WC). 1 The growing interest in ceramics for cutting tools is justi®ed from both an economical and technological perspective. Wear consists basically in a systemic phenomenon, which implies that each class of wear-resistant materials may ®nd use in dierent appli- cations, according to the counterpart of the tribological couple. Ceramics resist higher temperatures than metals without deforming. This allows tools to cut at faster speeds and deeper depths, resulting in increased removal rates and, consequently, cost ecient machining. 2 Recent developments in cutting tool ceramic materials include escalating improvements on the strength, frac- ture toughness, and wear-resistance of Al 2 O 3 and Si 3 N 4 . Albeit its high-strength and thermal-shock resistance, Si 3 N 4 has proven eective only for machining a limited number of materials, including cast irons and nickel- based alloys. 3 Furthermore, machining with Si 3 N 4 usually requires the use of coolants, which represent three times the current cost of cutting. On the other hand, Al 2 O 3 -based ceramic composites do not require cutting ¯uids yielding both economic and environ- mental bene®ts. 1 The development of new Al 2 O 3 -based composites has been accompanied by a signi®cant improvement in properties. This increases the range of applications for such materials as cutting tools, from widely used steels and cast irons to very hard steels and superalloys for the aerospace industry. 2 Aspects such as processing and microstructure have been extensively investigated. Fracture toughness, strength, hardness, and wear resis- tance have been particularly improved by the dispersion of hard carbide particles, such as TiC, 4 7 WC, 8 and NbC. 9,10 Moreover, the presence of dispersed particles can produce a pinning eect 11 and inhibit the grain growth of the matrix, which further contributes to the ®nal performance of the composite. 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00060-1 Journal of the European Ceramic Society 20 (2000) 1765±1769 * Corresponding author. Tel.: +55-84-215-3803; fax: +55-84-215- 3791. E-mail address: acchar@dfte.ufrn.br (W. Acchar).